Cambrian Explosion Month #31: Phylum Brachiopoda

While modern brachiopods superficially resemble clams, they’re not actually very closely related to each other. Clams are bivalve molluscs, related to snails and squid, while brachiopods are lophophorates related to bryozoans and horseshoe worms.

Their two shell valves are also arranged very differently – while bivalve shells originate from the left and right sides of their bodies, brachiopods grow theirs on the top and bottom.

They first appear in the fossil record in the early Cambrian, about 530 million years ago, but they may have actually diverged from a tommotiid-like ancestor as far back as the late Ediacaran. Only around 300 species survive today, but during the Paleozoic they were some of the most abundant filter-feeding and reef-building animals with tens of thousands of fossil species known. Different species tended to have strict habitat and temperature preferences, and so their fossils are also useful indicators of how ancient climates changed over time.

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Cambrian Explosion Month #17: Phylum(?) Vetulicolia & Other Early Deuterostome Weirdos

Vetulicolians were a group of odd Cambrian animals known from between about 520 and 505 million years ago. The front half of their bodies were large and streamlined, with a prominent mouth, no eyes, and five pairs of openings that seem to have been gills, with some species having a rigid exoskeleton-like carapace. Their back half was slender, segmented, and flexible, and functioned as a tail for swimming, giving them an overall appearance like alien tadpoles.

Their evolutionary affinities have been problematic for a long time, but evidence of a notochord in some specimens suggest they were probably related to the chordates in some way. Sometimes they’re considered to represent their own phylum, but they might also be stem-chordates or stem-tunicates.

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